1. Field of the Invention
This invention pertains to the field of power transmissions for vehicles, and more particularly to such systems that transmit power at the option of the vehicle operator, either to a first set of drive wheels or to first and second sets of drive wheels.
2. Description of the Prior Art
In current systems that provide part-time, four-wheel drive in a motor vehicle, the operator has to exit the passenger compartment in order to set the front wheel hublocks for four-wheel drive operation, then re-enter the vehicle and move the shift selector that controls the transfer case operation to the four-wheel drive position. Later manufacturers introduced full-time four-wheel drive systems that cause the front drive components to rotate at all times and thereby avoid any need for selectively operated hublocks. Full-time four-wheel drive systems, however, substantially reduce the fuel economy of the vehicle as compared to two-wheel drive operation because a large amount of inertial mass is continuously rotated. Recently, automatic hublocks, one associated with each of the wheels (usually the front wheels) that are driven on a part-time basis, were introduced to eliminate the need to leave the vehicle in order to engage the hublocks. In some of the part-time overdrive four-wheel drive systems, the vehicle must be stopped and shifted to four-wheel drive, then automatic hublocks are engaged when the vehicle is driven ahead. Hublock disengagement is accomplished the first time the vehicle is moved in the direction that is opposite to the direction that produced four-wheel drive operation; provided the transfer case is previously set for operation in two-wheel drive.
More recently, drive systems have been manufactured that permit shifting from two-wheel drive to four-wheel drive while the vehicle is moving. In such systems when two-wheel drive is selected, a sliding synchronizer collar within the front axle assembly disconnects one of the front axleshafts from the front differential allowing the ring gear of the differential, the front driveshaft and the transfer case chain to remain stationary while operating in two-wheel drive. In order to engage four-wheel drive, the operator moves the shift lever to the four-wheel drive, high speed ratio position. This action driveably connects the front driveshaft and allows it to accelerate to a speed that corresponds to the vehicle speed. Then a vacuum valve in the transfer case activates a vacuum diaphragm mounted in the engine compartment. The diaphragm pulls a cable, which moves a sliding collar in the front axle assembly to connect the differential to the previously disconnected front axleshaft. The shift from two-wheel drive to four-wheel drive when made in cold weather and under other adverse conditions when transmission oil viscosity is a factor, must be made only at slow speed in order to keep the effort required to move the shift lever at a reasonable magnitude. In four-wheel drive operation, shifts are made between the high speed ratio and low speed ratio provided the vehicle is stopped and the transmission is first shifted to neutral as in the conventional practice.